Methods, devices and systems, using ultrasonically activated passive sensors usable for sensing and measuring the values of different physical parameters within a human body or in other environments and scientific and industrial applications, have been described. U.S. Pat. No. 5,619,997 to Kaplan, incorporated herein by reference in its entirety for all purposes, discloses a passive sensor system using ultrasonic energy. An ultrasonic activation and detection system ultrasonically activates passive sensors having vibratable parts (such as vibratable beams or vibratable membranes) by directing a beam of ultrasound at the passive sensor or sensors. The sensor(s) may be implanted in a body or disposed in other environments. The activated passive sensor(s), or vibratable parts thereof, vibrate or resonate at a frequency which is a function of the value of the physical variable to be measured. The passive sensors thus absorb ultrasonic energy from the exciting ultrasonic beam mostly at the frequency of vibration (resonance frequency) of the sensor. The frequency (or frequency range) at which the passive sensor absorbs energy may be detected by a suitable detector and used to determine the value of the physical parameter.
The physical parameters measurable with such passive ultrasonic sensors may include, but are not limited to, temperature, pressure, the concentration of a chemical species in the fluid in which the sensor is immersed, and the thickness of a layer of substance deposited on the vibratable part of the sensor.
If the exciting ultrasonic beam is pulsed (or is an abruptly terminated CW beam), the ultrasonic sensor may continue to vibrate after the pulse (or the CW beam) terminates. The ultrasonic radiation emitted by the activated passive sensor after turning the exciting ultrasonic beam off may be detected and used to determine the value of the physical parameter of interest.
Since more than one physical variable may influence the vibration frequency of passive sensors, a correction may be needed in order to compensate for the effects of other physical parameters unrelated to the physical parameter which needs to be determined on the measured sensor vibration frequency. For example, if pressure is the physical parameter to be determined, changes in temperature may affect the vibration frequency of the sensor. U.S. Pat. Nos. 5,989,190 and 6,083,165 to Kaplan, both of which are incorporated herein by reference in their entirety for all purposes, disclose compensated sensor pairs and methods for their use for compensating for the effects of unrelated different physical variables on the determined value of another physical variable which is being determined. For example, such compensated sensor pairs, may be used for compensating for inaccuracies in pressure measurements due to temperature changes.
Typically, the size of implantable passive ultrasonic sensors, such as but not limited to intraluminal passive ultrasonic pressure sensors represents a design compromise. Decreasing the sensor's size may allow implantation in smaller intraluminal spaces and other body spaces and may decrease interference with blood flow or flow of other bodily fluids in intraluminal and other intra-cavity spaces. Smaller sensor size may also be advantageous in cases in which a number of sensors need to be implanted at the same location.
However, decreasing the sensor dimensions may also decrease the dimensions of the vibratable membrane which may decrease the energy absorbable from the exciting ultrasonic beam and the total energy radiated by the sensor at or about the resonance frequency. This may decrease the returned signal's strength and (assuming a fixed noise level) decrease the signal to noise ratio (S/N).
Additional problems which may be encountered with passive ultrasonic sensors when a narrow interrogating ultrasound beam is used to excite the passive sensor relate to the need to center the interrogating beam on the sensor in order to increase the S/N and the need to distinguish between the signal emitted by the sensor at the resonance frequency and spurious echoes reflected from reflecting surfaces other than the vibratable membrane of the sensor (such as, for example, echoes reflected from non-vibratable sensor surfaces and echoes reflected from various reflecting interfaces).